This invention, in its preferred form, relates generally to pumps, and, more particularly, canned pumps with high inertia flywheels.
Centrifugal pumps having flywheels are well known, the flywheel being incorporated to mechanically store potential energy during operation of the pump, which energy may be utilized to maintain rotation of the pump in the event of loss of motive power, such as loss of electric power. In nuclear reactors, this technology becomes very important to help maintain coolant circulation through the reactor core after coolant pump trip, since the nuclear fuel continues to give off substantial amounts of heat within the first several minutes after a reactor trip, and cooling is improved with forced flow. The flywheel is generally a metal disk having relatively high mass and being precisely attached to or mounted on the motor shaft for rotation therewith, the inertia of which keeps the shaft rotating after deenergization of the motor.
Pressurized water reactor (PWR) reactor coolant pumps generally include a pump and motor being separated by a complicated shaft seal system, the seals being used as part of the reactor coolant system pressure boundary. The seals are generally subject to about a 2500 psi pressure differential between the reactor coolant system and the containment atmosphere. These seals are susceptible to failure, and may cause a non-isolable leak of primary coolant ranging in size from very small to fairly large. As such, seal failure may result in a challenge to the redundant safety systems provided in nuclear power plants to prevent and mitigate damage to the reactor core.
Canned pumps have been used in nuclear reactor plants for some time, and avoid the problem of the shaft seal arrangement since the entire pump, including bearings and rotor, are submerged in the pumped fluid. Therefore, the use of the pump expressly reduces the potential for a small loss of coolant accident (LOCA). Exemplary canned motor pumps are described in U.S. Pat. Nos. 3,450,056 and 3,475,631. In boiling water reactors, continued rotation of these pumps upon loss of electric power is provided by electro-mechanical means, generally in the form of motor-generator sets having flywheels incorporated therein. The motor-generator set is generally located outside of the reactor containment for accessibility purposes, the electricity being transmitted from the generator to the pump motor through containment wall penetrations. In the event of a loss of electric power to the motor-generator set, the flywheel maintains rotation of the generator for some period of time, which continues to provide power to the pump motor. However, due to the lack of mechanical inertia in the pump itself, any localized failure of the pump or its controls may prevent the pump from extended coast-down. In addition, due to the necessity for extra equipment, this option becomes fairly expensive, both in capital cost and in operation and maintenance cost.
A flywheel within a canned or wet winding pump has been utilized. However, the losses resulting from spinning a large, high mass flywheel through the fluid contained in the pump casing are substantial. The outer surfaces of the flywheel attempt to frictionally pump the surrounding fluid, while the casing surrounding the flywheel inhibits fluid flow. Therefore, turbulent vortices form causing highly distorted fluid velocities which yields substantial drag on the flywheel. This drag is a function of the speed and area of the surface of the flywheel, which both increase with the radius of the flywheel, such drag being commonly understood to increase with about the fifth power of the diameter and about the cube of the angular velocity.
One arrangement to overcome this power loss is disclosed in U.S. Pat. No. 4,084,924 to Ivanoff et al. This patent describes a wet winding pump having a flywheel and a free-wheeling shroud rotatable relative to the shaft and the flywheel The shroud encompasses the flywheel but is spaced apart therefrom and includes passages for ingress and egress of liquid into and out of the space between the flywheel and the shroud. The disclosure envisions that the shroud will rotate at some angular velocity between zero and the velocity of the flywheel, thereby creating two pumped fluid layers, one (between the flywheel and the shroud) being pumped by the flywheel, and the other (the layer outside the shroud) being pumped by the shroud. The lower relative angular velocity between the rotating surfaces therefore results in lower total drag.
Therefore, it is the primary object of the present invention to provide a high-inertia flywheel for a canned or wet winding pump that minimizes the losses associated with the flywheel.